1. Field of the Invention
This invention relates to apparatus for holding a flexible sheet, and, more particularly, to apparatus holding a flexible sheet under tension while the flexible sheet is carried through chemical processes.
2. Background Information
Processes applied during the manufacture of flexible circuits, which are applied, for example, to polyimide sheets having a thickness of 0.002 inch (0.08 mm), require the holding and tensioning of such sheets as membranes having sides up to 14 inches (35.6 cm) long. Such circuits, and the sheets from which they are made, must be held as they go through one or more chemical process steps during manufacture.
Present methods for holding circuit sheets as they are carried through chemical processes include the use of double-sided adhesive tape, pins, springs, clamps, and screws. (Hereinafter, the term "circuit sheet" is meant to describe a flexible sheet being prepared for the application of circuit traces, a flexible sheet to which circuit traces are being applied, or a flexible sheet to which circuit traces have been applied). Double-sided adhesive tape used to fasten a flexible sheet or circuit to a frame member must frequently be replaced on the frame member, in an operation which is often difficult because the tape cannot easily be removed completely and cleanly. Furthermore, such tape frequently transfers an adhesive residue to the product being manufactured. Mechanical clamping methods tend to be complex, expensive to build, and difficult to maintain. Such mechanisms may collect and retain solutions from various chemical processes, resulting in a need for frequent and difficult cleaning, in corrosion problems, and, sometimes, in the contamination of one chemical process with residual materials from another such process. Furthermore, such conventional clamping mechanisms often require special features in the product being manufactured, such as clearance holes for clamping pins or screws. In some instances, difficulties are encountered because the thin sheet product is not clamped uniformly, being allowed to sag or billow between conventional clamping devices.
Thus, what is needed is a frame, to which a flexible sheet can easily be attached, and from which the circuit sheet can easily be removed, having a capability to maintain membrane tension within the circuit sheet as it is carried through various chemical processes. It is particularly desirable that such a frame be composed of materials resistant to chemical attack, and that such a frame be configured to avoid presenting surfaces including notches or cavities to the chemical processes through which the circuit sheet may be carried, since such features tend to collect chemical residue and are difficult to clean. It is further desirable to provide attachment means usable with a number of different types of circuit sheets without placing requirements on such sheets for special features, such as attachment holes. Such a frame is needed for the chemical processing of flexible circuit sheets and for the processing of other types of flexible sheets.